interesting question though I seem to believe that these definitions have to do mainly with:
the final categorie's approach speed defines the tax you pay on the approach and landing. typically the A321 should be CAT C for it's landing speed (142 kts) but is consideres B for the taxing
also the speeds are used to define approach construction protections. that's what is used to give you a racetrack return time that depends on your categorie. these times (if they're given in time and not in distance) usually depend on the categorie of the aircraft. same is true for an inversion procedure or a circling.
what you should do though depends on what procedure you're flying, i guess. if you're flying your racetrack at 180 kts it's good sense to consider the cat B times.
seb